The invention relates to a scandate cathode having a cathode body which comprises a matrix of a high-melting point metal and/or alloy, a barium compound in contact with the matrix material, to supply barium to the emissive surface by a chemical reaction with the matrix material, and a top layer of a Scandium containing material.
The invention also relates to methods of manufacturing such a cathode.
Cathodes of the type mentioned in the opening paragraph are described in the article "Properties and manufacture of top-layer scandate cathodes", Applied Surface Science, 26 (1986), pages 173-195, by J. Hasker, J. van Esdonk and J. E. Crombeen. In the cathodes described in this article scandium oxide (Sc.sub.2 O.sub.3) grains of several microns or tungsten (W) grains which are partially coated with either scandium (Sc) or scandium hydride (ScH.sub.2) are present at least in the top layer of the cathode body. The cathode body is manufactured by pressing and sintering tungsten grains, whereafter the pores between the grains are impregnated with barium-calcium-aluminate. The barium-calcium-aluminate supplies barium to the emissive surface by a chemical reaction with the tungsten of the matrix in order to maintain electron emission during operation of the cathode. During impregnation, in the cathodes manufactured with W which is partly coated with Sc or ScH.sub.2 the Sc is oxidized to Sc.sub.2 O.sub.3. In a very high load after mounting application in, for example, a cathode ray tube for television, it is important that a scandium-containing layer having a thickness of one monolayer be formed on the cathode surface during impregnation by means of a reaction with the impregnant. However, as has been proved in experiments described in the above-mentioned article, the scandium-containing layer may be partly or completely removed by ion bombardment which may occur during the manufacture of such television tubes, which detrimentally affects electron emission during later tube operation. Since Sc.sub.2 O.sub.3 is not very mobile, said scandium-containing layer cannot be fully regenerated by reactivation of the cathode. As compared with an impregnated tungsten cathode or an impregnant tungsten cathode coated with osmium-rhutenium or irridium, this may be considered as a drawback.